An image reading device generally comprises a light source and a light receiving portion. The light source emits light to irradiate an image-scanned region, when the reading device is operated. The light emitted from the light source is reflected at the image-scanned region of a document, which is a target of image reading. The light receiving portion receives the reflected light and outputs an image signal in accordance with the amount of light received. In such an image reading device, sometimes three light-emitting diode (LED) chips with different luminescent colors are applied as the light source. In this case, these LED chips are incorporated in the image reading device while being mounted in a predetermined wiring board.
FIG. 10A to FIG. 10C show three LED chips 60, 70 and 80 having different configurations that can construct the light source of the image reading device.
A first type of the LED chip 60 shown in FIG. 10A has a laminated structure comprising a P-type semiconductor layer 61a, N-type semiconductor layer 61b, and an active layer 61c formed therebetween. The lower surface of the laminated structure 61 in the figure is provided with an anode 62 covering substantially the whole area of the surface, and the upper surface is provided with a cathode 63 covering only a part of the upper surface. An LED chip having the configuration of the first type LED chip 60, for example, is employed as a red LED chip for the light source. When a predetermined voltage is applied through the anode 62 and cathode 63, light is emitted from the LED chip 60 via an exposed surface of the laminated structure 61.
A second type of the LED chip 70 shown in FIG. 10B has a laminated structure 71 and a transparent substrate 72. The laminated structure 71 comprises a P-type semiconductor layer 71a, N-type semiconductor layer 71b, and an active layer 71c formed therebetween. The transparent substrate 72 comprises a square-shaped portion and a frustum portion therebelow. On the lower surface of the laminated structure 71 is provided with an anode 73 covering substantially the whole area of the lower surface, and the upper surface 72a of the transparent substrate 72 is provided with a cathode 74 covering only a part of the upper surface. An LED chip having the configuration of the second type LED chip 70, for example, is employed as green and blue LED chips for the light source. When a predetermined voltage is applied through the anode 73 and cathode 74, light is emitted from the LED chip 70 via the upper surface 72a and inclined surfaces 72b of the transparent substrate 72 and via the side surfaces of the laminated structure 71.
A third type of the LED chip shown in FIG. 10C has a laminated structure 81 and a transparent substrate 82. The laminated structure 81 comprises a P-type semiconductor layer 81a, N-type semiconductor layer 81b, and an active layer 81c formed therebetween, and has a cutout portion 83. The P-type semiconductor layer 81a exposed to the cutout portion 83 is provided with an anode 84, and the lower surface of the N-type semiconductor 81b is provided with a cathode 85. An LED chip having the configuration of the third type LED chip 80 is sometimes employed as a blue LED chip for the light source. When a predetermined voltage is applied through the anode 84 and the cathode 85, light is emitted from the LED chip 80 via the upper surface of the transparent substrate 82 in the figure and via a predetermined side surface of the laminated structure 81.
FIG. 11 shows an example of a conventional mounting pattern of the wiring board of the LED chip for the light source in the image reading device. In the mounting pattern shown in FIG. 11, three LED chips 92R, 92G and 92B as the light source are mounted on a wiring board 91.
The LED chip 92R is a red light source and has the configuration of the abovementioned first type LED chip 60. The LED chip 92G is a green light source and has the configuration of the second type LED chip 70. The LED chip 92B is a blue light source and has the configuration of the second type LED chip 70.
Further, a plurality of photoelectric converters 93 as the light receiving portion, which are arranged in a line, are loaded on the wiring board 91. The wiring board 91 also has a wiring pattern 94 for configuring a circuit along with the LED chips 92R, 92G, 92B and the photoelectric converters 93. A predetermined place on the wiring pattern 94 is provided with three mounting pads 95 that are provided with respect to the LED chips 92R, 92G and 92B respectively, and with a connection pad 96, the LED chips 92R, 92G and 92B being loaded on the corresponding mounting pads 95.
FIG. 12A shows a conventional mounting structure of the wiring board 91 of the LED chip 92R, that is, the first type LED chip 60. When mounting the LED chip 60 on the wiring board 91, first, a chip bonder, for example, is used to press the LED chip 60 onto the mounting pad 95 by means of a solder or a conductive paste under a predetermined temperature condition, to join the anode 62 and mounting pad 95 of the LED chip 60 together via an adhesive metal portion 97 obtained originally from the solder or conductive paste. Next, by means of a wire bonding technology, the cathode 63 is electrically connected to the connection pad 96 via a wire W. In FIG. 12A, however, the adhesive metal portion 97 is not shown between the anode and mounting pad, for the purpose of simplification. The same thing can hold for FIG. 12B.
FIG. 12B shows a conventional mounting structure of the wiring board 91 of the LED chips 92G and 92B, that is, the second type LED chip 70. When mounting the LED chip 70 on the wiring board 91, first, the chip bonder, for example, is used to press the LED chip 70 onto the mounting pad 95 by means of the solder or conductive paste under a predetermined temperature condition, to join the anode 73 and mounting pad 95 of the LED chip 70 together via the adhesive metal portion 97 obtained originally from the solder or conductive paste. Next, by means of the wire bonding technology, the cathode 74 is electrically connected to the connection pad 96 via the wire W. In this manner, the LED chip 70 is mounted through the same steps as with the LED chip 60.
In the conventional mounting structure of the LED chip 60, the amount of the solder or conductive paste running over between the anode 62 and the mounting pad 95 in the mounting process is relatively large, thus, as shown in FIG. 12A, the adhesive metal portion 97 sometimes covers the side surfaces of the laminated structure 61 up to relatively high point of the surfaces. When applying a voltage to the LED chip 60, leak current easily runs through the place on the side surface that is covered by the adhesive metal portion 97, thus generation of the leak current causes a decrease in luminous efficiency or luminance of the LED chip 60. In addition, the fact that the adhesive metal portion 97 covers the side surfaces partially is also a cause of low luminance. In particular, when the adhesive metal portion 97 covers the side surfaces up to a point higher than the active layer 61c, the extent of the decrease in luminance is significant.
In the conventional mounting structure of the LED chip 70, the amount of the solder or conductive paste running over between the anode 73 and the mounting pad 95 in the mounting process is relatively large, thus, as shown in FIG. 12B, the adhesive metal portion 97 sometimes covers the side surfaces of the laminated structure 71 and a part of the inclined surfaces 72b of the transparent substrate 72. In the LED chip 70, the P-type semiconductor layer 71a is so thin that the active layer 71c is close to the mounting pad 95, thus a place involved in the side surface of the active layer 71c is completely and easily covered by the adhesive metal portion 97. When applying a voltage to the LED chip 70, leak current easily runs through the places on the side surface and the inclined surface 72b that are covered by the adhesive metal portion 97, thus generation of the leak current causes a decease in luminous efficiency or luminance of the LED chip 70. In addition, when the adhesive metal portion 97 covers the place involved in the side surface of the active layer 71c, it causes a significant decrease in luminance. Furthermore, the fact that the adhesive metal portion 97 partially covers the inclined surfaces 72b is also a cause of the low luminance.
FIG. 12C shows a conventional mounting structure when employing the third type LED chip 80 as the blue LED chip, instead of the second type LED chip 70. In order to mount the LED chip 80, instead of the mounting pad 95 and the connection pad 96 that are electrically connected to both electrodes of the LED chip 70, a wiring board 91′ on which different mounting pad 95′ and connection pad 96′ are formed.
When mounting the LED chip 80 on such a wiring board 91′, first, for example, the LED chip 80 is disposed on the wiring board 91′, such that the anode 84 of the LED chip 80 abuts on the mounting pad 95′ while the cathode 85 abuts on a part of the connection pad 96′. Next, an insulating resin adhesive 98 is filled between the LED chip 80 and the wiring board 91′. A solder or conductive paste cannot be used instead of the insulating resin adhesive 98 as a means to fix the LED chip 80 to the wiring board 91′. This is because if the solder or conductive paste is melted once and then solidified between the wiring board 91′ and LED chip 80, the anode 84 and cathode 85 short. Therefore, from a practical perspective, it is necessary to mount the third type LED chip 80 on the wiring board 91′ by means of a different method in which a material that is different from the first and second type LED chips 60 and 70 is used.
In the conventional mounting structure of the LED chip 80, the side surfaces of the laminated structure 81 are covered by the insulating resin adhesive 98, as shown in FIG. 12C. If the insulating resin adhesive 98 is fed between the LED chip 80 and wiring board 91′ at an amount sufficient to appropriately fix the LED chip 80 to the wiring board 91′ in the mounting process, a part of the insulating resin adhesive 98 runs over between the LED chip 80 and the wiring board 91′ and goes up the side surface of the laminated structure 81. The laminated structure 81 of the LED chip 80 is relatively thin, thus the part of the insulating resin adhesive 98 ends up covering the side surfaces of the laminated structure 81. Covering the side surfaces of the laminated structure 81 with the insulating resin adhesive 98 causes a decrease in luminance.
Moreover, when employing the third type LED chip 80 as the blue LED chip in the image reading device, instead of the second type LED chip 70, the conventional technology ends up causing an increase in the manufacturing man-hour and complicating the manufacturing line. This is because, in addition to the types of steps for mounting the LED chip 92R (the first type LED chip 60) and the LED chip 92G (the second type LED chip 70), a type of steps for mounting the blue LED chip (the third type LED chip 80) has to be performed, the type of steps being different from the above type of steps.
In addition, when employing the third type LED chip 80 as the blue LED chip in the image reading device, instead of the second type LED chip 70, in the conventional technology the wiring board 91′ instead of the wiring board 91 has to be prepared, the wiring board 91′ being different from the wiring board 91. Therefore, in a single manufacturing line of the image reading device, when employing both LED chips 70 and 80 as the blue LED chip, the wiring boards 91 and 91′ corresponding to the type of the LED chips are necessary, according to the conventional technology. This is not preferred in terms of the production cost and the management cost.